1. Field of Invention
The present invention relates generally to data storage devices and more specifically to a hardware independent system and method for adaptively managing and controlling multi-path access to an array of data storage devices.
2. Description of Related Art
Large-scale integrated database management systems provide an efficient, consistent, and secure means for storing and retrieving vast amounts of data. This ability to manage massive amounts of information has become a virtual necessity in business today. Indeed, in some cases, the data stored in the company's databases may provide the very backbone for the company, as in stock fund trading and reservation systems. In those businesses, and others, the data may assume the status of a business strategic asset.
As businesses begin to rely more heavily on large scale database management systems, the consequences of hardware-related data losses intensify, and the security, reliability, and availability of those systems becomes paramount.
One way to increase the security, reliability and availability of data stored in large databases is to employ a technology known as a redundant array of inexpensive disks, or RAID. RAID is a storage approach that achieves high availability by applying redundancy techniques to the storage of data. These redundancy techniques can also improve input/output (I/O) performance by re-distributing access to storage across the multiple storage devices decreasing queue lengths and improving access time. This process is known as load-balancing.
High data availability is often achieved through redundancy, either by "mirroring" or by the use of "parity" drives. Mirroring involves a one-for-one backup of the data storage devices for each drive, allowing the data to be simply read off of the remaining mirrored drive when the primary drive fails. The parity technique augments the array of storage devices with an additional storage device that contains the parity of the data from corresponding logical locations of the RAID rank (the number of drives that the data and parity will be spread across). If a drive in a RAID rank is lost, the controller reconstructs the lost data from the remaining data and the parity data, preventing the loss of data.
Five RAID "levels" are commonly used. RAID 0 provides increased throughput, but does not increase reliability, since it does not provide for additional redundancy. Large files are striped across multiple drives in chunks (a predetermined multiple of the logical block size) to improve data access and reduce data latency. RAID 1 provides full mirroring of each disk. If one of the disks fail, the mirrored copy takes over until the failed disk is replaced. Performance can be improved, if, in addition to mirroring, reads are spread across the spindles of the mirrored disks as well. In RAID 3, data is stored across multiple drives, and a parity value is generated and stored on a dedicated parity storage device. If one of the primary storage devices fails, the data can be reconstructed from the remaining disks and the parity disk. RAID 5 is similar to RAID 3, except that the parity data is spread across all drives in the array.
While current RAID technology provides increased availability and reliability through control of redundant storage elements in a disk array, it does not take full advantage of system-level redundancies. For example, although a server/disk array combination may include redundant paths to one or more storage devices in the disk array, or paths to other storage arrays, current systems do not permit the global, system-wide monitoring or control of data across these paths. One reason for this deficiency is the inherent difficulty in implementing a systems-level storage solution that is operable with a variety of disk storage array designs and technologies.